Various types of lithium electrochemical cells in non-aqueous solvents are known in the art. Primary solid cathode lithium cells typically include a lithium anode, an electrolyte prepared from lithium salts dissolved in one or more organic solvent and a cathode containing electrochemically active materials such as transition metal oxides, metal sulfides, fluorinated carbon compounds, etc.
One of the drawbacks of such prior art lithium cell arises from the highly reactive nature of the lithium metal in air. Lithium readily reacts with water vapor in air. Therefore, lithium anodes must be prepared in an entirely dry atmosphere. The preparation of metallic lithium anodes is therefore cumbersome, expensive and may also be hazardous.
Another drawback of commercially available solid cathode primary lithium cells is that their operating voltage varies in a range of 1.5-3.3 Volts. There are currently no primary lithium cells based on metallic lithium with a solid cathode that operate at 3.5-4.1 Volts.
Another drawback of primary lithium cells is encountered in high-power primary lithium cell designs where a thin metallic lithium anode is required. A common problem in such high power cells is the low tensile strength of metallic lithium. The preparation of metallic lithium anodes may therefore require the use of excess lithium in the anode to increase the thickness of the lithium in the anode (in order to provide better mechanical strength), or the incorporation into the anode of an electrically conducting support such as a metallic or a metalized supporting foil or supporting mesh (for example, a copper or nickel foil or mesh or another metal plated with gold or chromium or the like, may be used to increase the anodes mechanical strength) or another suitable electrically conducting support or the like. The use of such a conducting support (onto which the lithium is plated or deposited or attached), suitably increases the anode's mechanical strength.
The first approach (excess lithium) markedly reduces the practical energy density (available energy per volume unit) achievable by the cell. The second approach (using a thin conducting support) may markedly complicate the anode manufacturing process because a vacuum deposition method or other similar manufacturing methods may have to be used to deposit the thin layer of metallic lithium on the conducting support. Such techniques are inefficient for mass production processes, may require costly equipment and may have to be performed in batch.
One approach to overcome the low operating voltage problem encountered with the currently available primary lithium cells is to use cathode materials such as transition metal oxides (or transition metal chalcogenides) in combination with carbonaceous anodes based on graphite or petroleum coke capable of intercalating lithium ions. In using this approach, lithium ions have to be removed from the lithiated cathode by an externally applied charging current and intercalated into the carbonaceous anode.
This approach, while increasing the cell's operating voltage, has two main drawbacks. The first drawback is a very high self-discharge rate of the resulting cells (typically about 5% of the cell's charge per month). While such a high self-discharge rate value may be commercially acceptable for rechargeable lithium cells, it is not acceptable for most of primary lithium cells for which a loss of up to 0.1% of the cell's charge per month is typically required. The second drawback of commercially available high-voltage lithium cells is the low energy density as compared to primary lithium cells. The main reason for this low energy density arises from the low theoretical capacity value of the carbonaceous anode in comparison to a lithium metal anode. Such carbonaceous anodes may deliver up to 372 mAh/gr while lithium metal anodes may theoretically provide values of 3860 mAh/gr.
As for rechargeable electrochemical lithium cells, various types of non-aqueous rechargeable lithium cells are known in the art. Rechargeable lithium cells, such as the cells described in U.S. Pat. No. 4,828,834 (Nagaura at al), incorporated herein by reference in its entirety for all purposes, include a highly electroactive metallic lithium based anode, a lithium salt, organic solvents and an electrochemically active cathode. In such cells, during discharge, lithium ions pass from the anode through the liquid electrolyte and are intercalated into the cathode. During the charging of the cell, the flow of ions is reversed. Lithium ions pass from the cathode through the electrolyte and are deposited back as metallic lithium atoms on the lithium anode. The quality of the lithium layer deposited or plated on the anode during the charging of the cell is not good enough for many charge discharge cycles. This kind of lithium deposition tends to yield a high surface area plating form known as dendrites. Such dendrites typically continue to grow upon cycling of the cell. Unfortunately, lithium dendrite formation limits the number of permissible charging/discharge cycles, as eventually the dendrites may contact the cathode which may result in cell failure. Dendritic lithium formation in rechargeable cells may thus make such cells inherently less stable since if such a cell short-circuit occurs, the cell may explode.
Moreover, the high-surface area dendritic lithium on the anode's surface tends to react with the electrolyte to form an electrically isolated non-active substance. As a result, the amount of the remaining lithium available in the cell decreases, reducing the practically achievable energy density of the cell.
It may be possible to partially overcome this low efficiency resulting from the low quality of the lithium plating during the charging half-cycle by including a large excess of lithium metal in the cell (typically a four fold excess-as compared to the practical capacity of the cathode). However, using excess of lithium in the cell increases the thickness of the anode and therefore undesirably decreases the practically achievable energy density of the cell. Moreover, using a larger quantity of lithium is inherently more dangerous, decreasing overall cell safety, and, as lithium is a comparatively expensive metal, increasing the cell's cost.
A different approach used to improve the number of charge/discharge cycles is to use a rechargeable cell having a carbonaceous anode as described in U.S. Pat. No. 4,423,125 (Basu et al.), incorporated herein by reference in its entirety for all purposes, and in U.S. Pat. No. 5,028,500 (Fong et al.), incorporated herein by reference in its entirety for all purposes. These cells include a carbonaceous anode including a suitable carbon form such as coke or graphite intercalated with lithium ions to form LixC6 where X<1. As taught by Fong et al., typical graphite compositions will take up between 0.5 and 1 mole of lithium for each 6 moles of carbon included in the carbonaceous anode composition.
At X=1, the maximum theoretical capacity of graphite is only 372 mAh/g graphite in comparison to 3860 mAh/gr for pure lithium metal. As noted by Basu et al., deposition of lithium on carbon beyond Li1C6 tends to be highly reactive with organic electrolyte solvents, which are typically used in lithium cells. The ensuing side reactions may lead to lithium loss in the anode and may ultimately cause cell failure. Thus, to quote from Basu et. al. “Such freshly reduced elemental lithium on an anode surface tends to be highly reactive with organic electrolyte solvents which are typically used in lithium batteries. Such side reactions lead to the loss of lithium from the anode and can cause ultimate cell failure. Thus, by substantially reducing their presence one can increase the rechargeability of such a battery”. It is thus clear that the deposition of highly reactive lithium metal on the carbonaceous anode of such prior art lithium cells is problematic.
Another approach to increase the energy density of rechargeable lithium cells beyond the energy obtained with intercalated carbon is described in U.S. Pat. No. 5,576,119 to Yamin et al), incorporated herein by reference in its entirety for all purposes. Yamin et al. disclose a rechargeable electrochemical cell having an anode including a thin layer of electrically conductive material such as copper or nickel and a cathode including a lithiated metal oxide on an aluminum supporting foil. Lithium deposition on the anode is accomplished in-situ during the first charge of the cell. The drawback of this approach is the relatively low number of charge/discharge cycles attainable that results from the poor quality of lithium metal deposition on the surface of the conductive material of the anode.